1. Field of the Invention
The present invention relates to a semiconductor device. More specifically, the present invention relates to a semiconductor device having a pin structure wherein a semiconductor region having a low impurity concentration is arranged between a p-type semiconductor region and an n-type semiconductor region.
2. Description of Related Art
A PiN diode is generally employed as a semiconductor power device having a high reverse blocking voltage. As is shown in FIG. 1, a PiN diode has an n−-type base layer 101, a p-type emitter region 103 connected to one of the main surfaces of the n−-type base layer 101, an n+-type emitter region 107 connected to the other opposite main surface, an anode electrode 108 connected to the p-type emitter region 103, and a cathode electrode 109 connected to the n+-type emitter region 107.
A positive voltage, relative to the cathode electrode 109, is applied to the anode electrode 108. Holes are injected from the p-type emitter region 103 to the n−-type base layer 101, and in accordance with the amount of injected holes, electrons are injected from the n+-type emitter region 107 to the n−-type base layer 101. The holes and electrons (hereinafter referred to as “carriers”) are accumulated in the n−-type base layer 101, and the resistance of the n−-type base layer 101 is reduced. The diode is then rendered conductive, and a current flows from the anode electrode 108 to the cathode electrode 109.
When a voltage applied in the conductive state is inverted, the carriers accumulated in the n−-type base layer 101 are discharged, and a depletion layer starts to spread, beginning at the pn junction between the n−-type base layer 101 and the p-type emitter region 103. As a result, the diode falls into a blocking state.
A PiN diode according to related art may further include an n-type or n+-type semiconductor region arranged between the n−-type base layer 101 and the p-type emitter region 103, and implements a soft recovery characteristic for a reverse recovery.
A PiN diode according to other related art further includes an embedded control electrode that reaches inside the n−-type base layer 101, so that the efficiency of the injection of carriers is improved and a voltage in the forward direction in the conductive state is reduced.
Since a switching frequency was required to be increased recently according to the demand that the efficiency of an inverter, for example, should be improved, a reverse recovery loss for the diode must be reduced. In order to reduce the reverse recovery loss, the amount of the carriers accumulated in the n−-type base layer 101 needs to be reduced. To reduce the amount of the carriers, only the impurity concentration of the p-type emitter region 103 needs to be lowered. However, to maintain low contact resistance between the p-type emitter region 103 and the anode region 108, the impurity concentration on the contact surface of the p-type emitter region 103 can not be reduced. Therefore, any reduction in the impurity concentration of the p-type emitter region 103 is limited and thus, a reduction in the reverse recovery loss of the diode has been limited so far.
In addition, once the impurity concentration of the p-type emitter region 103 is reduced, the number of the carriers accumulated in the n−type base layer 101 is reduced when driven by a small current. Therefore, at the time of the reverse recovery, the depletion layer spreads quickly, and the voltage rising rate is increased. This high voltage rising rate causes the breakdown voltage of the load to deteriorate.